Weather Systems. The air around high-pressure weather systems tends to swirl in a clockwise direction, and usually brings clear skies.

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1 Weather Systems A weather system is a set of temperature, wind, pressure, and moisture conditions for a certain region that moves as a unit for a period of several days. Low-pressure weather systems form when a warm air mass and a cold air mass meet. The air around such systems tends to swirl in a counterclockwise direction due to the Coriolis effect, and usually brings cloudy skies and stormy weather. The air around high-pressure weather systems tends to swirl in a clockwise direction, and usually brings clear skies. Thunderstorms A thunderstorm is a storm with lightning, thunder, heavy rain, and sometimes hail. Thunderstorms result from the rapid upward movement of warm, moist air. Generally, thunderstorms require three things in order to form: 1. Moisture is needed to form clouds and precipitation. 2. An unstable air mass. 3. A lifting force (heat). All thunderstorms, regardless of type, go through three stages: the developing stage, the mature stage, and the dissipation stage. The average thunderstorm has a diameter of 24 km. Developing Stage In this stage, a warm air mass and a cold air mass collide. The warm air rises upward (this is known as an updraft), carrying moisture with it. As the warm air cools, the moisture within it condenses to form a cumulus cloud. The cloud will continue to grow as long as warm air below it continues to rise. Mature Stage As the cumulus cloud becomes larger, the water in it becomes larger and heavier. Raindrops start to fall through the cloud when the rising air can no longer hold them up. Meanwhile, cool dry air starts to enter the cloud. Because cool air is heavier than warm air, it starts to descend in the cloud (this is known as a downdraft). The downdraft pulls the heave water downward, making rain. The cloud has now become a cumulonimbus cloud, because it has an updraft, a downdraft, and rain. Thunder and lightning start to occur, as well as heavy rain. The cumulonimbus is now a thunderstorm cell.

2 Dissipating Stage After about 30 minutes, the thunderstorm begins to dissipate. This occurs when the downdraft in the cloud begins to dominate over the updraft. Since warm moist air can no longer rise, cloud droplets can no longer form. The storm dies out with light rain as the cloud disappears from bottom to top. The whole process takes about one hour for an ordinary thunderstorm. Supercell thunderstorms are much larger, more powerful, and last for several hours. The diagram below illustrates the stages in thunderstorm formation.

3 Tornadoes A tornado is a violently rotating column of air that is in contact with both the surface of the Earth and a cumulonimbus cloud. Tornadoes form in the most severe thunderstorms. Fast rising air in a thunderstorm sometimes starts spinning, forming a funnel of air and moisture. As more rising air is replaced by cooler air at the surface, the rotation gets faster and faster. The rising air causes a pressure difference that increases with speed. This pressure difference is capable of lifting roofs, cars, trailer homes, and people, sending them flying through the air. The intensity of a tornado is rated according to the Fujita scale, as shown below.

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5 Worksheet 1. Thunderstorms are associated with a specific type of cloud. Name it. 2. Thunderstorms require three conditions in order to form. What are they? 3. All thunderstorms go through three stages. Name these three stages. 4. What is the diameter of the average thunderstorm? 5. Briefly summarize the first stage in the life of a thunderstorm. 6. The second stage of a thunderstorm s life is marked by the presence of what two things simultaneously?

6 7. Briefly summarize the final stage in the life of a thunderstorm.